Gallium Arsenide (GaAs)

Gallium arsenide is a hard (~750 HV Knoop), extremely brittle III-V semiconductor compound with a density of 5.32 g/cm³ and a melting point of 1238°C. It cleaves readily on {110} planes, making fracture the primary concern during preparation. The material is toxic due to arsenic content; all debris must be collected as hazardous waste. GaAs is typically examined as single-crystal wafers or polycrystalline substrates for dislocation density, precipitates, twin boundaries, and surface damage from device processing.

Precision wafering only. Use a low-speed diamond wafering saw with a thin (0.15-0.3 mm) diamond blade. Cutting speed: 100-200 RPM with very low feed rate. The extreme brittleness and {110} cleavage tendency require minimal mechanical force during sectioning. Clamp gently using padded fixtures to prevent fracture. Continuous coolant flow is essential for both thermal control and arsenic dust containment. Standard abrasive cut-off wheels are NOT suitable; they will shatter the sample. For wafer samples, scribing and controlled fracture along crystal planes may be preferred over sawing.

Cold mounting with castable epoxy is required. Use a low-viscosity, low-shrinkage epoxy resin to ensure complete encapsulation and edge retention. Vacuum impregnation is recommended, especially for samples with cracks or device layers that need preservation. Compression mounting must be avoided; the pressure and temperature will fracture the brittle GaAs. For cross-section analysis of device structures, orient the sample carefully in the mount to expose the layer of interest. An edge-retaining epoxy formulation is essential for preserving thin surface layers.

The extreme brittleness requires very gentle grinding with minimal applied force. Use bonded diamond grinding discs rather than loose-abrasive SiC paper to reduce subsurface damage. Disc speed: 150-250 RPM. Apply very light pressure (10-15 N per 30 mm sample). The material will fracture or chip if pressure is excessive.

Grinding sequence:

• 30μm diamond disc: Remove sectioning damage (30-60 seconds). Very light pressure. Monitor edges for chipping.
• 15μm diamond disc: Refinement (30-60 seconds). Continue very light pressure.
• 9μm diamond disc: Final grinding (30-60 seconds). Prepare for polishing.

SiC paper can be used as an alternative (320, 600, 1200 grit) but generates more subsurface damage in brittle semiconductors. Diamond discs are strongly preferred. Rotate specimen 90° between steps. Use water-based lubricant. Collect all grinding waste as hazardous material.

Use firm, napless polishing cloths to prevent edge chipping and maintain flatness. The brittle nature means any relief or uneven support can cause fracture at thin edges.

Diamond polishing sequence:

• 6μm diamond: 3-5 minutes on a napless hard pad with light pressure (10-15 N per 30 mm sample). Use oil-based diamond suspension to prevent chemical attack from water.
• 3μm diamond: 3-5 minutes on a napless pad (10-15 N). Continue with oil-based suspension.
• 1μm diamond: 2-4 minutes on a napless pad (8-12 N).

Final polishing:

• 0.05μm colloidal silica: 2-4 minutes on a soft pad with very light pressure (8-10 N). Alternatively, a chemo-mechanical polish using dilute bromine-methanol (0.5-1% Br in methanol) on a soft cloth for 1-2 minutes provides excellent results by combining chemical dissolution with mechanical polishing to remove subsurface damage.

All polishing waste containing arsenic must be collected for hazardous waste disposal.

GaAs responds to several chemical and electrolytic etchants. Etching reveals dislocations, grain boundaries, twin boundaries, and crystal defects.

Bromine-Methanol (Chemical Etching) - Standard for GaAs:

• Composition: 1-5% bromine in methanol (1-5 ml Br₂ per 100 ml methanol)
• Application: Immerse for 10-30 seconds. Agitate gently. Higher concentrations etch faster.
• Reveals: Dislocations (as etch pits on {100} surfaces), grain boundaries, twin boundaries, and crystal defects. The standard general-purpose etchant for GaAs.
• Rinse: Methanol, then dry with warm air or nitrogen.
• Safety: Bromine is highly toxic and corrosive. Work in a fume hood. Use glass or PTFE containers only.

Citric Acid-H₂O₂ (Chemical Etching) - Gentler alternative:

• Composition: Saturated citric acid solution mixed 1:1 with 30% H₂O₂
• Application: Immerse for 15-60 seconds. Very controllable etch rate.
• Reveals: Layer structures, etch-stop interfaces, and general microstructure with less aggressive attack than bromine-methanol.

Safety: All etching waste contains dissolved arsenic and must be disposed of as hazardous waste.